Glass optical fibers are typically manufactured from solid “preform” rods, which are essentially large-scale models of the eventual fiber. That is, the geometry of the preform—the core, the surrounding cladding and any additional internal structures—will be retained in the fiber, which is “drawn” from the preform. This is accomplished by lowering the preform into a furnace at the top of a tower. In the furnace, tightly controlled temperatures (usually approaching 2000° C.) soften the tip of the preform. Once the softening point of the preform tip is reached, gravity takes over and a molten swell of material free-falls until it has been stretched into a thin strand. The operator threads this strand of fiber through a series of coating dies and measurement stations, and the drawing process begins. The fiber, which retains the relative internal dimensions and relationships of the preform, is pulled by a capstan situated at the bottom of the draw tower and then wound on winding drums.
Optical fiber preforms are typically manufactured using one of the following processes: modified chemical vapor deposition (MCVD), outside vapor deposition (OVD), vapor axial deposition (VAD), or plasma vapor deposition (PVD). MCVD is one of the most commonly used processes. In general, MCVD involves the use of a heated glass tube that is rotated by a lathe while chemicals are conducted into the glass tube in a vapor phase.
More specifically, with reference to FIG. 1A, the MCVD process involves a glass tube 10 that is rotated by a lathe 12 at a constant rate. A torch 20 producing a flame repeatedly travels longitudinally from one end of the glass tube 10 to the other and back. A heat control unit 24 controls the temperature of the torch 20, by varying the mixture of O2 and H2 provided to the torch 20. As the tube 10 is rotated and heated, various chemical compounds in the vapor phase are carried by oxygen into the glass tube 10 through a gas feed pipe (conduit) 28, which is in communication with a connector 32 at one end of the glass tube 10. The compounds may be conducted from a plurality of bubblers 36a, 36b, 36c (referred to generally as bubbler 36) and can include, for example, SiCl4, which is used to generate SiO2, a main constituent of the optical fiber material; GeCl4, used to obtain GeO2 for controlling a refractive index of the fiber core or cladding; and POCl3. The rates at which the compounds enter the glass tube 10 are controlled by mass flow controllers 38a, 38b, 38c, 38d (referred to generally as mass flow controller 38), each of which controls the flow of a carrier gas, for
As the compounds enter the glass tube 10, they react in the interior region of the tube which is heated by the flame from the torch 20. The predominant reaction is SiCl4+O2→SiO2+2Cl2. The SiO2 is deposited on the wall of the glass tube 10 in the solid phase, as soot particles, and Cl2 and any unreacted compound is exhausted from the glass tube 10 in a gaseous state. Typically, as illustrated by FIG. 1B, the soot particles follow the heat gradient and deposit ahead of the flame. Layer upon layer of material is deposited within the glass tube 10 as the torch 20 successively passes over the axial length of the tube 10.
Often it is desirable to form a fiber preform with dopants other than those typically used in the MCVD process (i.e., P, Ge, B, Ti). For example, a lasing dopant (e.g., a lanthanide such as Er, Yb or Tm) may be introduced into the fiber preform. Unfortunately, lasing-dopant precursors such as Er compounds are typically used in solid form, and therefore do not lend themselves to MCVD because they do not exhibit sufficient vapor pressure. Therefore, to introduce these compounds into the depositing material within the glass tube, the soot is unsintered or only partially sintered—that is, the flame is maintained at a temperature (1100–1400° C., for example) insufficient to fully sinter the soot into glass, so that it remains porous. The tube 10 is subsequently removed from the lathe and immersed in a solution of the desired dopant. The dopant soaks into the partially sintered soot. Then, after soaking, the glass tube is placed back in the lathe, dried and reheated at a higher temperature (approximately 1800° C. is typical), thereby sintering the dopant-soaked soot particles into a doped glass preform.
The foregoing process is inconvenient and cumbersome in requiring unsintered deposition followed by soaking and then re-application of heat to sinter the dopant-containing material. Moreover, this approach may also result in uneven doping, soot inconsistencies, flaking, or cracking and consequent commercial unsuitability of at least part of the final product.
Another method for fabricating doped optical fiber preforms includes forming a glass core on the inner surface of a quartz reaction tube according to the MCVD technique. A compound of a rare earth element (i.e., a dopant) is located in a chamber formed at one end portion of the quartz reaction tube, where it is heated with a burner and sublimated. The vapor-phase reactants used to produce the core are introduced into the quartz reaction tube along with the sublimated dopant compound, thereby causing deposition of a core glass doped with the rare earth element.
Ordinarily, the vapor pressure of the rare earth element compound is sufficiently low that the compound is liable to settle out. Accordingly, the concentration of the doped rare earth element is apt to become non-uniform especially along the length of the optical fiber preform. Another limitation is the limited doping level that can be achieved. In addition, it is difficult to accurately control the concentration of the doped rare earth element.
Still another method of producing a doped optical fiber preform is described in U.S. Pat. No. 5,711,782 to Okamura et al. (“Okamura”), the entire contents of which are herein incorporated by reference. The method of Okamura involves cooling to an appropriate temperature the glass tube in which the gaseous chemicals are reacted, and moving a dopant solution feed pipe at a given speed along the length of the reaction tube while spraying an atomized dopant solution from a nozzle. The atomized dopant solution is sprayed over the porous, soot-like core glass and impregnated therein. The movement of the dopant solution feed pipe at a given speed ensures uniform impregnation of the solution along the length of the reaction tube.
After dehydration, the porous core glass is consolidated (i.e., sintered). Finally, the glass layers within the reaction tube are collapsed to obtain an optical fiber preform. The process of Okamura requires two heating steps, one to partially sinter the core and a second to fully sinter (i.e., consolidate) the core after doping.
Accordingly, there exists a need for method of fabricating a doped optical fiber preform without interrupting the MCVD process to dope the preform.